This invention relates generally to electrical water heaters. More particularly, this invention pertains to heating elements for electrical water heaters used to heat water containing hardness values which tend to coat heating surfaces with scale.
Conventional electric water heaters have elongated heating elements comprising an outer tubular sheath enclosing an inner electrical resistance wire. The resistance wire is connected at each end of the element to electrical terminals in a flange or other mount for electrical activation. Typical element designs include at least one return bend with a short radius enabling passage of the element through an entry port. Additional bends may be provided to lengthen the element and increase the heating surface area.
In a typical element, the internal metallic resistance wire is surrounded by a material such as magnesium oxide which is an electrical insulator but is capable of a reasonably high heat transfer rate. The outer sheath may be formed of a metal such as copper or INCOLLOY material. Thermal energy passes from the hot resistance wire through the insulating material and sheath wall to the sheath surface, thereby heating the water.
It is theoretically desirable to design the element for a high heat evolution, measured as "watt density", i.e. units of power per unit sheath external heat transfer area.
In nearly all uses of water heaters, the water contains precipitatable chemical compounds measured as "hardness". These compounds, including calcium sulfate, typically precipitate on the hot sheath surfaces, forming a heat insulative scale comprising salts of sulfates, carbonates, oxides, etc.
In the absence of significant scale on the sheath, the heat transfer mechanism keeps the electrical resistance wire at a relatively low temperature. As a layer of scale accumulates on the sheath surface, the resistance to heat transfer increases rapidly, and the temperature of the resistance wire, magnesium oxide and sheath increases. The deleterious effects of such scale-induced elevated element temperatures are well-known, and include:
a. decreased heat transfer rate; PA1 b. increased rate of scaling at the higher temperatures; PA1 c. "burn-out" of the resistance wire due to oxidation and melting at the high temperatures; PA1 d. cracking or breaking of the sheath due to high temperature stress; and PA1 e. the required frequent replacement of the heating elements.
Scale accumulation is significantly greater at sharp bends in the element. The sheath area for heat transfer is reduced at the interior portion of the bends, resulting in higher temperatures in this area. The rate of scale formation at bends is significantly greater than in straight areas, and the scale eventually fills the interior portion of the bend. The result is very high element temperatures at the bends. Aggravating this problem are the increased stresses and potential surface cracking resulting from the bending operation in these areas.
Various solutions have been proposed or used to allay the problems created by scaling of heating elements.
In one method, the watt density is reduced so that the scale will form at a lower rate, thus extending the element life. This may be accomplished by using a resistance wire of lower wattage rating, or increasing the sheath diameter and/or length. The disadvantages of this method are that an element of greater surface area is required, causing difficulties in fitting the element into small heater tanks and/or increasing the cost through (a) enlarged element size and (b) enlarged port and element mount size and greater required strength thereof.
Another method for reducing scaling problems comprises the use of elements having greater-than-normal watt density. The element is intended to heat very rapidly when turned ON so that the element expands rapidly, thereby "flaking" off the scale from the sheath surface. This method sometimes works, depending upon the chemical structure of the scale. It has been observed that even using such a method, a high degree of scaling will eventually occur. The increased watt density makes the element less tolerant of scale, i.e. the element temperature rises more rapidly per unit thickness of scale, resulting in high element temperatures. Failure of the element typically occurs very prematurely.